The attack of sulfur compounds on metals at elevated temperatures is an exceedingly important phenomenon is petroleum refining. Compounds naturally occurring in crude petroleum and other compounds formed during the processing of the oil may corrosively attack at various places within the processing equipment. Typical sulfur compounds include H.sub.2 S, sulfides, disulfides, mercaptans, etc. The mechanism by which attack of metals by sulfur compounds occurs is not well understood. A typical result of what will be called herein "sulfidation attack" is the formation of a thick porous scale of iron sulfide which typically, being loose, tends to spall, thus exposing fresh surface. The scale itself has little or no protective effect and cannot be relied upon to reduce corrosion as is often the case in other metals where the initial layer of corrosion products will effectively protect the base metal beneath.
The consequences of such corrosive attack are severe both in terms of initial capital expenditures for expensive alloy equipment and in replacement costs for corroded equipment. Also, it is often the case that forced shutdowns of refinery processes occur earlier than expected due to equipment failure caused by sulfur compound corrosion. Such shutdowns, of course, result in large additional losses over and above those incurred by replacement of the equipment itself. The total cost of sulfur corrosion to the petroleum industry is not known but must represent a very large expense which must be borne by the refiner. Thus, mitigation of sulfur corrosion is of extreme importance to the petroleum refining industry. The solution to this problem has generally been approached by attempting to find improved metals or alloys which will successfully resist the aggressive attack of sulfur compounds. Generally iron alloys containing chromium and nickel have been used, since it has been found that adding chromium and nickel to steel provides protection against sulfur attack. It is necessary to build equipment of or lined with such alloys in order to prevent excessive corrosion in areas where it has been found that such attack occurs extremely rapidly. Unfortunately, the severity of corrosion attack and the exact location where it will occur cannot be predicted accurately, but is done empirically, judging from the quantity of sulfur present in the oil and from the conditions which exist in the equipment. Since crude oils contain differing amounts of sulfur and the sulfur compounds present differ from one to another, the corrosion which will result from processing of crude oils has not been predictable. Consequently, it has been found that equipment is either protected insufficiently, leading to premature failures and excessive replacement costs, or, more expensive materials are used in constructing refining equipment which later experience shows unnecessary in view of the corrosion actually experienced.
Another problem which results from corrosive attack by sulfur compounds is the fouling of equipment by the iron sulfide scales which form during the corrosion process and later spall off, fouling and plugging downstream equipment and causing unscheduled shutdowns.
Heretofore, no suitable method has been found to add protection for carbon steel, iron-chromium alloys, and iron chromium-nickel alloys, against aggressive attack by sulfur compounds. All that has been done is to predict the degree of corrosion that would be involved and use a material containing a sufficient quantity of chromium and nickel in order to give a satisfactorily low corrosion rate. Then, the equipment so protected would have a reasonable life expectancy and unscheduled and expensive shutdown would be avoided. The present invention discloses a novel method of producing a protective film on iron and iron alloys which may be used to significantly reduce the corrosion rate which would be otherwise experienced and thereby providing a longer useful service life for the equipment and reducing the cost of sulfidation attack.